This paper presents the experience made with the engineering and execution of the tie-in of flexible risers to rigid pipelines on a project recently completed in West Africa.
Five production and injection pipelines (10” and 6”) were tied back to the host platform with flexible risers, in Lazy wave configurations, in ∼600m water depth.
The risers are directly connected to the terminations structures (PLETs) of the rigid pipelines, through horizontal connection systems.
The structures forming the tie-in (risers, PLETs and pipelines) have been designed to accommodate axial displacements of the pipelines in the range 0.3m to 1.0m, as positive displacements (expansions) and −0.1m to −0.7m as negative displacements (contractions) of the pipelines, respectively towards and away from the risers, due to pipelines thermal expansions and pipe walking.
Note that along some of the lines anchoring structures have been installed to control pipe walking.
The tie-in interface loads were to be limited, in order not to threaten the flexible pipe, the PLETs and the connectors, and, despite the small pipeline end displacements, keeping the interface loads within allowable values, was a challenge.
The positive displacements were causing interface loads as high as 80% of the allowable values, while the negative displacements were causing up to 90% utilization of the capacity of the connectors and 95% of the allowable loading of the foundations of the PLETs.
The main drivers of such high loadings are the stiffness of the flexible pipe, combined with the layout of the tie-in.
Extensive in place analyses were done to simulate the effects of progressive displacements of the pipelines, the pipe-soil interactions and the specifics of the behaviour of the flexible pipes (hysteretic stiffness).
Full 3D FE analyses of the foundations (mud mats) of the PLETs were done, to circumvent the limitations of a classical bearing capacity analysis approach.
As built information were also used, to remove some conservatisms in the assumptions initially taken in the design. A special installation procedure was implemented, to achieve a layout of the riser at the approach of the pipeline capable to better relieve the displacements of the pipelines and reduce interface loads.
Feedbacks from the installation are given in the paper.
The lessons learned are also presented: a “flexible” pipe is a “stiff” structure and a direct tie-in to the pipeline may become an issue, if not addressed early enough during the execution of the project, when it can be too late to add mitigation structures, like intermediate tie-in spools, or to change significantly the routing of the risers and pipelines.